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previous: BWilliamson Been digging all night thinking abo... -- 6/7/2000 11:39 AM

Re: Power Tranny help needed on Marshall clone?

6/7/2000 7:44 PM
Carl Z	Re: Power Tranny help needed on Marshall clone? I doubt that those numbers are guesses. All you need to do is call the manufacturer and give them the part number and they'll tell you the rating. They won't give you design data...that's proprietary information. One reason there's not a lot of ratings published is that it'll just confuse the buyer if he/she doesn't know exactly what they're looking at...case in point! A higher rated transformer will yeild a much stiffer supply so for something like hi-fi applications they require much more supply regulation, particularly at low frequencies, than a guitar amplifier. A nice thumb rule for getting the right transformer is to find the ratings that the manufacturers use and work from there. Example; You're building a knock off 50 watt plexi. Marshall's trannies are rated at 690@150mA. So we go to the Hammond catalog and see what they've got. You find a listing for a transformer that specs out at 660@200mA(arbitrary numbers). Well, based on our experience with Marshall units we can guess that our B+ voltage will be a little lower but because of the increased current capacity of the transformer we can also expect that B+ won't drop as far under full power. This might not actually be the case though. Due to copper losses, heating, transformer efficency etc., the B+ values might wind up a little higher or even lower. Ain't this fun! The best you can do is make sure your transformer isn't under rated with respect to the original specs and see what other peoples results are then go from there. Here's why these numbers don't look quite right. First off the values for my equipment are at the endpoints of the power supply load line. They're the most extreme value's you'll ever see under any circumstances. This is not to say that a Fender or Marshall won't draw as much current as mine. Most likely they will. These rating might also be based on a baseline output level at a certain THD. The reason the production transformer ratings are lower is COST! The major costs to the end user in manufacturing a transformer are the lamination material, thickness and the windings. If you go with a larger core and/or heavier copper windings to increase the current capacity of the transformer the costs will go up significantly. The engineer's job is to try to strike a reasonable compromise between cost and performance. The designer figures that the amp is NOT going to be run with a 1 volt input signal with all the controls on 10, at least not for long enough to do any real damage to the transformer. So, to save 5 bucks or so per transformer they derate it to it's expected operating range. Based on production runs upward of 1000 units this can be a substantial savings! Fender 50 watt amps running at 5% THD are only putting out around 42 watts. At these numbers they'll draw maybe 200mA from the power supply, probably much less. They don't expect the user to run these flat out so they manufactured their trannies to handle 240mA at full load. Given that these transformers are still running great after 40 years of serious abuse you can see that they really didn't need to use a transformer rated much higher than that. Here's the rub. Most people using Fender amps don't generally use these amps at their full output, but they damn sure run Marshalls flat out all the time. This is probably why their trannies fail more often. One spec that's frequently overlooked is the current rating on the heater windings. I've seen more blown Fender trannies due to shorted or open heater windings than anything else. They just figure that they can toss in a set of EL34's and they'll have a killer OD tone. WRONG! The heater winding in most Fenders will tolerate a set of EL34's...barely. The biggest problem with Marshalls and hotrodded Fenders is the rating of the OPT. A 50 watt amp will require a transformer rated for much higher wattage than that. A square wave output (heavy distortion) will present the load with roughly double that value! A Fender or Marshall transformer rated at 60 watts will never hold up for any length of time. And we all know how often these transformers croak! Hope this helps clear things up a little. Carl Z

6/7/2000 7:44 PM

Carl Z

Re: Power Tranny help needed on Marshall clone?

I doubt that those numbers are guesses. All you need to do is call the manufacturer and give them the part number and they'll tell you the rating. They won't give you design data...that's proprietary information.

One reason there's not a lot of ratings published is that it'll just confuse the buyer if he/she doesn't know exactly what they're looking at...case in point!

A higher rated transformer will yeild a much stiffer supply so for something like hi-fi applications they require much more supply regulation, particularly at low frequencies, than a guitar amplifier. A nice thumb rule for getting the right transformer is to find the ratings that the manufacturers use and work from there.

Example; You're building a knock off 50 watt plexi. Marshall's trannies are rated at 690@150mA. So we go to the Hammond catalog and see what they've got. You find a listing for a transformer that specs out at 660@200mA(arbitrary numbers). Well, based on our experience with Marshall units we can guess that our B+ voltage will be a little lower but because of the increased current capacity of the transformer we can also expect that B+ won't drop as far under full power. This might not actually be the case though. Due to copper losses, heating, transformer efficency etc., the B+ values might wind up a little higher or even lower. Ain't this fun!

The best you can do is make sure your transformer isn't under rated with respect to the original specs and see what other peoples results are then go from there.

Here's why these numbers don't look quite right. First off the values for my equipment are at the endpoints of the power supply load line. They're the most extreme value's you'll ever see under any circumstances. This is not to say that a Fender or Marshall won't draw as much current as mine. Most likely they will. These rating might also be based on a baseline output level at a certain THD.

The reason the production transformer ratings are lower is COST! The major costs to the end user in manufacturing a transformer are the lamination material, thickness and the windings. If you go with a larger core and/or heavier copper windings to increase the current capacity of the transformer the costs will go up significantly. The engineer's job is to try to strike a reasonable compromise between cost and performance. The designer figures that the amp is NOT going to be run with a 1 volt input signal with all the controls on 10, at least not for long enough to do any real damage to the transformer. So, to save 5 bucks or so per transformer they derate it to it's expected operating range. Based on production runs upward of 1000 units this can be a substantial savings!

Fender 50 watt amps running at 5% THD are only putting out around 42 watts. At these numbers they'll draw maybe 200mA from the power supply, probably much less. They don't expect the user to run these flat out so they manufactured their trannies to handle 240mA at full load. Given that these transformers are still running great after 40 years of serious abuse you can see that they really didn't need to use a transformer rated much higher than that.

Here's the rub. Most people using Fender amps don't generally use these amps at their full output, but they damn sure run Marshalls flat out all the time. This is probably why their trannies fail more often.

One spec that's frequently overlooked is the current rating on the heater windings. I've seen more blown Fender trannies due to shorted or open heater windings than anything else. They just figure that they can toss in a set of EL34's and they'll have a killer OD tone. WRONG! The heater winding in most Fenders will tolerate a set of EL34's...barely.

The biggest problem with Marshalls and hotrodded Fenders is the rating of the OPT. A 50 watt amp will require a transformer rated for much higher wattage than that. A square wave output (heavy distortion) will present the load with roughly double that value! A Fender or Marshall transformer rated at 60 watts will never hold up for any length of time. And we all know how often these transformers croak!

Hope this helps clear things up a little.

Carl Z

Replies:

BWilliamson Thanks Carl--I think this is seepin... -- 6/7/2000 9:30 PM
JaySG Re: Heater rating Fender PT -- 6/7/2000 11:56 PM